1. Field of the Invention
The present invention relates to an optical transmission apparatus. More particularly, the present invention relates to an optical transmission apparatus in which a selected one of light outputs with different wavelengths emitted from a plurality of light sources can be modulated with a selected one of a plurality of data signals.
2. Description of the Related Art
In the field of optical communication, various techniques are known. For example, in Japanese Laid Open Patent Disclosure (JP-A-Showa 63-299538), an optical switch circuit is disclosed. Also, in Japanese Laid Open Patent Disclosure (JP-A-Heisei 1-99336), an optical transmission apparatus is described in which replacement of an optical semiconductor element is made possible without stopping communication. In Japanese Laid Open Patent Disclosure (JP-A-Heisei 1-114128), a wavelength multiple optical communication system is disclosed in which a loop is provided in a light path in such a manner that detection of a fault part or a test can be easily performed. In Japanese Laid Open Patent Disclosure (JP-A-Heisei 2-67525), a transmitting method between an optical switch apparatus and an optical switch is described. In Japanese Laid Open Patent Disclosure (JP-A-Heisei 2-219330), a wavelength multiple light transmission system is described in which light outputs from the plurality of light sources are synthesized by an optical coupler of a plurality of stages.
Also, in Japanese Laid Open Patent Disclosure (JP-A-Heisei 4-115732), an optical transmission apparatus is described in which a range of dispersion degradation to be compensated can be expanded. In Japanese Laid Open Patent Disclosure (JP-A-Heisei 8-251113), an optical transmission apparatus is described in which the variation in phase between multiplexed light signals is devised to cancel sensitivity degradation.
However, in the above references, it is not described that an arbitrarily selected one of the light outputs from light sources can be modulated with an arbitrarily selected one of the data signals. Also, it is not described that the light intensity of the modulated signal can be set to a predetermined level at that time.
An optical transmission apparatus in which a combination of one of a plurality of light signals and a single data signal can be optionally set is described in U.S. Pat. No. 5,245,681 (a first conventional example). In this conventional example, one optionally selected light signal from among light outputs with different wavelengths respectively emitted from a plurality of light sources is modulated with a single data signal by a single optical modulator to produce a light signal.
FIG. 1 is a block diagram illustrating the structure of the first conventional example of optical transmission apparatus. In the first conventional example of optical transmission apparatus, 4 light sources are used.
Referring to FIG. 1, continuous wave (CW) light outputs with different wavelengths are outputted from first to fourth light sources 202.sub.1 to 202.sub.4. The light outputs from the first to fourth light sources 202.sub.1 to 202.sub.4 are supplied to the input ports of an optical switch 214. The optical switch 214 is a (4.times.1) optical switch which is composed of 2-stage (2.times.1) optical switches. The optical switch 214 outputs one selected from among the light outputs from the four light sources 202, to 202, in accordance with control signals from an optical switch control circuit 217. The selected light output is modulated in accordance with a data signal from a modulator driving circuit 216 in the optical modulator 215 and is outputted onto an optical fiber or a transmission path as a light signal.
The optical switch control circuit 217 controls a control circuit 218 in addition to the optical switch 214. The control circuit 218 controls the first to fourth driving circuit 201.sub.1 to 201.sub.4 such that each of the light outputs from the first to fourth light sources 202.sub.1 to 202.sub.4 has a predetermined optical power level.
When the first conventional example of the optical transmission apparatus is applied to the wavelength division light transmission apparatus for N wavelengths, there is the following problem. That is, in the first conventional example of the optical transmission apparatus shown in FIG. 1, if it is required that an arbitrary combination of one of the N light outputs emitted from the N light sources and one of the N data signals can be selected, then N of the same optical system units are needed. That is, (N.times.N) light sources, N modulators and N optical switches are needed. Accordance, the apparatus structure becomes very complicated and becomes expensive.
FIG. 2 is a block diagram illustrating the structure of a second conventional example of an optical transmission apparatus using typical optical modulators. In the second conventional example of the optical transmission apparatus, one of light outputs with different wavelengths emitted from N (N is an integer equal to or more than 2) light sources is modulated with one selected from among data signals through electrical switching into a light signal by optical modulators, respectively.
The light outputs with the different wavelengths emitted from the first to N-th light sources 302.sub.1 to 302.sub.N are inputted to first to N-th optical modulators 304.sub.1 to .sup.304.sub.N and are modulated based on N data signals 301.sub.H to 30N.sub.H. As a result, N light signals 1.sub.0 to N.sub.0 are outputted from the first to N-th optical modulators 304.sub.1 to 304.sub.N, respectively. To select a combination of the light output emitted from each light source and one of the electric data signals as modulation signals, a (4.times.4) electric signal switching unit 319 needs to be provided between the data signals and the modulator driving circuits 305.sub.1 to 305.sub.N.
However, in the second conventional example of the optical transmission apparatus shown in FIG. 2, because only N light sources and N modulators are needed, the structure of the optical system can be simplified. However, the data signal inputted to the optical modulator must be switched by the electric signal switching unit 319 when the modulation is performed in the combination of a light output from a light source with a wavelength and the data signal. In this case, when the transmission rate of the data signal is high, the switching between the data signals becomes difficult.